An archer's bow is a simple machine in which the limbs define a two-arm spring. An arrow consists of a forward tip which may be a target point type or a broadhead type affixed to one end of a shaft which is typically made from wood, fiberglass, metal or other suitable material, a nock for resting against a bowstring, and fletching, also known as fins or vanes, which are affixed to the shaft just ahead of the nock for purposes of aerodynamic stabilization during flight. The archer stores energy in the form of the drawn stressed bow. When the archer releases the bowstring permitting the bow limbs to spring forward kinetic energy is then transferred to the arrow. Among several factors affecting the distance an arrow flies are the initial angle, initial velocity, arrow weight, length of the arrow, and the size and shape of the arrow fletching. Spin influences directional stability which is the stability of a moving body about an axis. Drag stability is directional stability produced by the fletching on the arrow shaft. Rate of spin is determined by vane geometry and more specifically to the fletching scheme which may be straight, offset or helically oriented. Both offset and helical configurations will cause the shaft to spin; with a helical fletching configuration producing a relatively higher rate of shaft rotation.
When preparing to shoot an arrow, the nock of the shaft is temporarily mounted to the bowstring which is then drawn back, deforming the bow which acts as a store of potential energy. Conventional (fixed) nocks are typically one-piece and attached to one end of the arrow shaft. A fixed nock possesses a static bowstring rest that when engaged with the bowstring, prevents the arrow shaft from assuming a spin induced by the fletching during the initial release phase of the arrow which is the time from bowstring release by the archer to bowstring separation from the nock. Accordingly, with fixed nocks, it is only after the nock separates from the bowstring that rotation of the arrow shaft can begin to occur. Thus, a conventional nock (1) robs the arrow of energy by immobilizing the arrow shaft and preventing fletching rotation when moving through the air, which produces drag on the arrow during the initial release phase, and (2) interferes with early stabilization that would occur at the onset of release if the arrow were somehow permitted to begin spinning during the initial release phase.
What is needed is a nock assembly that permits natural rotation of the fletching during the initial release phase by allowing the rotation of the shaft imparted by the fletching configuration moving through the air to occur immediately after the archer releases the drawn bowstring—and prior to separation of the nock from the bowstring. Such a nock would (1) reduce wind resistance by allowing the fletching to promote a natural spin of the arrow immediately upon bow string release, (2) increase stabilization of the shaft by permitting early spin and (3) eliminate string torque which is caused by non-uniform forces present when portions of a fixed nock contacting the moving bowstring are forced angularly against the bowstring due to the natural tendency of helical fletching to attempt rotation when moving forward. Because the nock is radially torqued against the bowstring by the rotation of the fletching acting on the shaft, the torquing introduces destabilizing forces to the arrow shaft. In some cases, the arrow after separating from the bowstring and immediately after leaving the bow will attempt to maintain the rotational direction imparted by the torqued string and can be seen to reverse its rotation. This torquing effect has been confirmed by slow motion video. Finally, for at least the reasons given above, a nock permitting the free rotation of an arrow shaft when still engaged with a bowstring should, assuming the same shooter and gear, provide a relatively greater degree of accuracy.
Various devices in the past have struggled with the problem of promoting arrow spin; typically once the nock separates from the bowstring. However, many such devices have included springs or spiraled guides that interfere with the natural tendencies of helical fletching to rotate the arrow shaft to assume a natural rotational equilibrium consistent with fletching geometry and other physical factors present at release, i.e., mass of the arrow, density of air, and the thrust imparted by the bow. It is known that the faster an object spins, the greater the inertia. Accordingly, induced rotation exceeding natural rotation robs energy from the bow which reduces kinetic energy available for forward motion of the arrow. Still another problem with devices that artificially increase rotation is the straining of the bowstring rest portion of the nock torquing against the bowstring when thrust forward by the released bowstring. In cases of artificially inducing a rotational velocity exceeding that which would otherwise occur if the nocked arrow shaft were passively allowed to commence rotation when moving through the air, the moving arrow shaft is destabilized by increasing air turbulence around the fletching during the initial release phase; after which, the rotational rate experiences a correction by air resistance acting on the fletching slowing the rotation. The correction to artificially induced rotational speed can be sudden. In the case of a mass encountering a resistive fluid at a velocity beyond which the fluid can efficiently accomodate, much turbulence is produced as molecules of the fluid collide with each another and the moving mass. In other words, the more turbulence produced, the greater the destabilizing forces acting on the arrow shaft. When natural rotation is permitted to occur, the air molecules pass less chaotically around the fletching and allow the arrow to move forward in a relatively smooth trajectory.
It would be desirable to promote a natural spin of an arrow shaft by the rotation of fletching during the initial acceleration phase of an arrow's release.
It would be desirable to reduce the drag upon the fletching of an arrow during the initial acceleration phase of an arrow's release and from that time immediately after the initial acceleration phase when the arrow separates from the bowstring until the fletching is able to adequately rotate the shaft.
It would be desirable to increase the travel for a released arrow.
It would be desirable to increase the stability of an in-flight arrow.
It would be especially desirable to eliminate torquing of the nock relative to the bowstring at the instant of nock-bowstring separation, by providing a freely rotatable nock which provides low-friction rotation of the nock when still engaged with the bowstring.
In keeping with the foregoing, it would be desirable to improve the accuracy of arrow flight.